Of course, the most rewarding part is the "Aha" moment, the excitement of discovery and enjoyment of understanding something new, the feeling of being on top of a hill, and having a clear view. But most of the time, doing mathematics for me is like being on a long hike with no trail and no end in sight! I find discussing mathematics with colleagues of different backgrounds one of the most productive ways of making progress.
Recorded Clay Mathematics Institute annual report interview, Clay Mathematics Institute Annual Report 2008, 1 January 2008 source